feat: agent proactively re-engages users on returning sessions

Two complementary changes to reduce conversation cold-start friction:

A. Returning-user re-greeting (backend)
- When WS reconnects to a non-empty conversation and last_message_at is older
  than chat_re_greeting_idle_hours (default 6h), the agent emits a warm
  continuation message that references prior history instead of staying silent.
- Self-debouncing: the AI message updates last_message_at, so reconnects
  within the window will not re-trigger.
- Skipped while profile collection is still pending.

D. Topic suggestion chips (backend + Expo)
- New WS message type topic_suggestions carries 3-4 quick-start chips derived
  from the current memoir stage's empty slots (deterministic, no extra LLM
  cost). Sent alongside opening / re-greeting / resume.
- Expo chat screen renders a horizontally-scrollable chip row above the input
  bar; tapping a chip sends the chip's text as a user message and clears the
  row. Sending any text/voice also clears the chips.

api/app/agents/chat/orchestrator.py

@@ -477,3 +477,26 @@ class ChatOrchestrator:
             profile_birth_year=profile_birth_year,
             profile_era_place=profile_era_place,
         )
+
+    async def generate_re_greeting_message(
+        self,
+        conversation_id: str,
+        memoir_state: MemoirStateSchema,
+        idle_hours: float,
+        user_profile_context: str = "",
+        background_voice: str = "default",
+        occupation: str = "",
+        profile_birth_year: Optional[int] = None,
+        profile_era_place: str = "",
+    ) -> List[str]:
+        """委托 InterviewAgent 生成老对话回访问候（持久化由调用方负责）。"""
+        return await self.interview_agent.generate_re_greeting_message(
+            conversation_id=conversation_id,
+            memoir_state=memoir_state,
+            idle_hours=idle_hours,
+            user_profile_context=user_profile_context,
+            background_voice=background_voice,
+            occupation=occupation,
+            profile_birth_year=profile_birth_year,
+            profile_era_place=profile_era_place,
+        )